1. Field of the Invention
The present invention relates to a ZrO2—Al2O3 composite ceramic material having high mechanical strength and toughness as well as excellent wear resistance and hardness under a relatively larger Al2O3 content than before, and a production method therefor.
2. Disclosure of the Prior Art
As compared with metal materials and plastic materials, ceramic materials have excellent performance such as hardness, wear resistance, heat resistance and corrosion resistance. However, for the actual use of the ceramic materials in wide application fields such as mechanical parts for automobiles, airplanes, space crafts and so on, cutting tools including drills and surgical knives, medical device, and biomaterial parts such as artificial joints and artificial tooth, it is desired to develop a ceramic material having good balance between mechanical strength and toughness in a high level. As one of potential candidates of such a ceramic material, attention is being given to a ZrO2—Al2O3 composite ceramic material.
For example, Japanese Patent Early publication [kokai] No. 5-246760 discloses a ZrO2-based ceramic composite material having improved mechanical strength and toughness. This ceramic material is composed of a matrix phase of tetragonal ZrO2 grains containing 5 to 30 mol % of CeO2 and a dispersion phase of fine grains of at least one selected from the group of Al2O3, SiC, Si3N4 and B4C, which are dispersed within the ZrO2 grains and grain boundaries of the ZrO2 grains. By the presence of the dispersion phase, a grain growth of the matrix phase can be prevented, so that there are advantages that a fine structure of the matrix phase is obtained, the stabilization of tetragonal ZrO2 is enhanced, and a reduction in flaw size is achieved.
In addition, U.S. Pat. No. 5,728,636 discloses a ZrO2-based ceramic material having high mechanical strength and toughness. This ceramic material comprises a first phase of partially stabilized ZrO2 grains having an average grain size of 5 μm or less, and containing 8 to 12 mol % of CeO2 and 0.05 to 4 mol % of TiO2, and a second phase of Al2O3 grains having an average grain size of 2 μm or less. A content of the second phase in the ceramic material is within a range of 0.5 to 50 vol %. The Al2O3 grains are dispersed within the ZrO2 grains at a dispersion ratio of 2% or more, which is defined as a ratio of the number of the Al2O3 grains dispersed within the ZrO2 grains relative to the number of the entire Al2O3 grains dispersed in the composite ceramic material.
In addition, this ceramic material is produced by mixing a first ingredient for providing the partially stabilized ZrO2 grains containing the above described ranges of CeO2 and TiO2, with a second ingredient for providing the Al2O3 grains, molding a resultant mixture into a desired shape to obtain a green compact, and then performing pressureless sintering of the green compact in the oxygen-containing atmosphere. In this US patent, by use of the required amounts of CeO2 and TiO2 as the stabilizer for ZrO2, the grain growth of the ZrO2 grains is enhanced to a moderate degree, so that parts of the Al2O3 grains are allowed to effectively disperse within the ZrO2 grains, and a critical stress is increased, derived from a stress-induced phase transformation from the tetragonal ZrO2 to the monoclinic ZrO2.
However, since the grain growth of ZrO2 is inhibited as the content of Al2O3 in the ceramic material increases, there is a tendency of reducing the number of the Al2O3 grains dispersed within the ZrO2 grains, i.e., the dispersion ratio described above. The reduction in dispersion ratio may deteriorate the balance between the mechanical strength and toughness. On the other hand, as the Al2O3 content increases, further improvements in hardness and wear resistance of the ceramic material are expected. Therefore, if a ZrO2—Al2O3 composite ceramic material containing a relatively large amount of Al2O3 as compared with the conventional ones can be provided, which has excellent wear resistance and hardness brought by the increased Al2O3 content, while maintaining the good balance between the mechanical strength and toughness, commercialization of the ceramic material in the many application fields described above will be further promoted.